Known as examples of a hydraulic pump and a hydraulic motor are an axial piston hydraulic pump and an axial piston hydraulic motor. Examples of the axial piston hydraulic pump are a swash plate type hydraulic pump and a bent axis type hydraulic pump. Examples of the axial piston hydraulic motor are a swash plate type hydraulic motor and a bent axis type hydraulic motor. For example, a pump disclosed in PTL 1 is known as a swash plate type hydraulic pump (hereinafter may be simply referred to as a “swash plate type pump”). Moreover, for example, a motor disclosed in PTL 2 is known as a swash plate type hydraulic motor (hereinafter may be simply referred to as a “swash plate type motor”). Further, for example, a pump motor disclosed in PTL 3 is known as a bent axis type hydraulic pump motor.
Each of these pumps and motors include a valve plate. The configurations of the pump and motor are basically the same as each other except that: in the pump, a cylinder block is rotated by the rotation of a driving shaft; and in the motor, a motor shaft is rotated by the rotation of a cylinder block. The valve plate will be explained using the swash plate type pump of PTL 1 as an example.
FIG. 11 shows a swash plate type pump 61 of PTL 1. A cylinder block 64 fixed to a driving shaft 63 and capable of rotating together with the driving shaft 63 is included in a pump housing 62 of the swash plate type pump 61. A rear end surface of the cylinder block 64 contacts a valve plate 65 to be supported by the valve plate 65. A plurality of cylinders 66 are formed on the cylinder block 64 so as to be located around the driving shaft 63 and be parallel to one another. Pistons 67 are respectively inserted in the cylinders 66. Tip end portions of the pistons 67 are respectively coupled to shoes 67a. The shoes 67a are rotatable together with the cylinder block 64 and the pistons 67 and are slidable with respect to a shoe plate 68 fixed to a swash plate 69.
When the driving shaft 63 is rotated by a driving device, not shown, the cylinder block 64 also rotates, and the pistons 67 reciprocate in the cylinders 66 by a reaction from the swash plate 69. The rear end surface of the cylinder block 64 is pressed against the valve plate 65 by the action of internal pressure of the cylinders 66. Since the cylinder block 64 rotates in this state, frictional heat is generated on sliding surfaces of the valve plate 65 and the cylinder block 64. Generally, while sealing the operating oil by the sliding surfaces, lubrication and cooling are also performed by an appropriate amount of drain oil (leakage oil). Thus, thermal balance is maintained. However, the seizure of the sliding surfaces or the thermal crack of the valve plate 65 may occur due to the increase in the internal pressure of the cylinders 66 or the increase in the rotation speed of the cylinder block 64. If the amount of leakage oil is increased for the purpose of increasing the cooling effect, the efficiency of the pump or the motor decreases,
The same problem as above occurs in the bent axis type pump using the valve plate. Further, the swash plate type motor and the bent axis type motor, each basically having the same configuration as the hydraulic pump, cannot avoid the same problem as above.